DESCRIPTION (applicant's abstract): Behavioral studies in mammals demonstrate that REM sleep is important to subsequent performance of many memory tasks. The idea that sleep (particularly rapid eye movement, or REM sleep) could serve a cognitive function has remained popular since Freud stated that dreams were "not nonsense" but a time to sort out experiences and discharge emotions. Others have proposed that REM sleep is for forgetting. We seek to determine whether neurons in the hippocampus, a structure critical to certain types of learning and memory, discharge at random during REM sleep or, instead, show ordered activity differentially modified by prior experience. The hippocampus is particularly amenable to such inquiry, since the ensemble-firing pattern of hippocampal neurons is thought to form a cognitive map and control the learning of new landmark relationships. Recordings from individual cells while rats actively navigate their environment reveal place-specific firing (any particular neuron is mostly silent, but discharges robustly at a particular area in the environment, the location of which varies from cell to cell). Preliminary results indicate that experience affects not only the firing rate, but also the pattern of neural activity during REM sleep. The phase-firing pattern, in turn, indicates a mechanism through which memories for recent events may be strengthened and familiar events weakened during REM sleep. This proposal aims to determine the physiological turning point at which novel memories become familiar in the temporary memory store of the hippocampus and test these patterns under conditions that disrupt and enhance memory consolidation. This effort will contribute to the knowledge base of memory consolidation mechanisms from short to long-term stores, so that we may ultimately devise effective ways to improve memory and learning capabilities of impaired individuals.